Variable/torque speed a.c. induction machines (motors and generators) are generally, conventionally controlled according to one of two strategies. One strategy uses a fixed slip frequency selected to provide maximum torque at maximum voltage and varies the voltage in order to obtain variations in the torque/speed. The system is simple and fairly effective, but suffers from low efficiencies, especially at torques other than the maximum torque obtainable at the selected slip frequency. The second strategy varies the slip frequency proportionally to the required torque in order to attempt to emulate a constant saturation flux operation of the machine. At high torques this approach provides reasonable efficiencies, but at lower torques, due to the efficiency characteristics of the motor, the efficiency is poor. In addition the second strategy does not permit operation of the machine above its rated speed.